1. Field of the Invention
The present invention relates to an audio signal amplifier circuit and an audio device using such circuit, and, more specifically, relates to an improvement in an amplifier circuit which converts digital audio signals to analogue audio signals and amplifies the same for a portable audio device such as a portable magnetic tape player, a portable CD player and an audio player making use of an IC memory (an IC memory audio player), which permits to reduce power consumption in the circuit, to operate in a high efficiency and to prolong the playing time of a battery.
2. Background Art
A conventional audio device including a headphone stereo, a portable magnetic tape player for such as DCC and DAT, a portable disk player for such as MD and CD and an IC memory audio player now under development is driven by a battery, and number of batteries therefor is usually about one or two. Therefore, the power source voltage for such audio circuit is only about 1.2V or 2.4V. Moreover, in most cases such device is provided with a stereo function, thus the output amplifier circuit thereof is constituted in two series.
For these portable type audio device, demands for reducing number of driving power source batteries into a single battery and for realizing a longer playing time with a single battery are keen. Further, in such recent circuit a low pass boost amplifier is provided which tends to increase the power consumption.
Portions which consume large electric power in these circuits or the devices are an audio output circuit and a motor device circuit. For example, JP-A-4-111506 (1992) and JP-A-7-222976 (1996) disclose reduction of operating current and suppression of power consumption in an audio circuit. In the former, input signals are applied to a separate power source series other than a signal amplifier series and are full-wave rectified, and the operating current of an audio output circuit is controlled by the full-wave rectified signals, thereby, an electric power depending on the amplitude of the input signals is supplied to the audio output circuit. In the later, an imaginary grounded center amplifier portion when amplifying stereo signals is replaced by a switching regulator to generate a voltage of an imaginary grounded level, thereby, a reduction in power consumption is achieved.
In these sorts of conventional audio amplifier circuits, in particular, such as for a CD or MD portable disk player and an IC memory audio player, digital signals are converted into analogue signals, the converted analogue signals are amplified and dynamic speakers therefor are driven by the amplified analogue output signals. Further, such as in a portable magnetic tape player, reproduced analogue signals are amplified in an analogue form and the speakers are driven by the amplified analogue output signals.
In order to amplify these analogue signals and to drive the dynamic speaker with these analogue output signals, a preamplifier and a sound volume adjusting circuit are required which are operable with signals of from microscopic outputs to power outputs and, thus amplification of audio signals has to be repeated several times. Accordingly, number of capacitors and resistor circuits increases as well as the number of transistors also increases, and the power conversion efficiency up to sound generation remains at most about 60%.about.70%. It is impossible right now to expect increasing the efficiency more than the above referred to percentage.
Accordingly, a reduction of battery power consumption is limited to a certain level as well as even if the size of concerned circuits is reduced by making use of ICs, the thinning and miniaturizing of the devices are limited.